The C-terminal domain of Eos forms a high order complex in solution

Ikaros family transcription factors play important roles in the control of hematopoiesis. Family members are predicted to contain up to six classic zinc fingers that are arranged into N- and C-terminal domains. The N-terminal domain is responsible for site-specific DNA binding, whereas the C-terminal domain primarily mediates the homo- and hetero-oligomerization between family members. Although the mechanisms of action of these proteins are not completely understood, the zinc finger domains are known to play a central role. In the current study, we have sought to understand the physical and functional properties of these domains, in particular the C-terminal domain. We show that the N-terminal domain from Eos, and not its C-terminal region, is required to recognize GGGA consensus sequences. Surprisingly, in contrast to the behavior exhibited by Ikaros, the C-terminal domain of Eos inhibits the DNA-binding activity of the full-length protein. In addition, we have used a range of biophysical techniques to demonstrate that the C-terminal domain of Eos mediates the formation of complexes that consist of nine or ten molecules. These results constitute the first direct demonstration that Ikaros family proteins can form higher order complexes in solution, and we discuss this unexpected result in the context of what is currently known about the family members and their possible mechanism of action.